1. Field of the Invention
The present invention relates to a position measuring instrument for determining the absolute position and to a method for absolute position measurement.
2. Discussion of Related Art
In many fields, for determining the position of two bodies moved toward one another, absolute position measuring instruments are increasingly being used. Absolute position measuring instruments have the advantage over systems that measure purely incrementally that in every relative position, even after the energy supply has been interrupted, a correct position information can be output immediately.
The absolute position is embodied by a code, which is arranged in a plurality of tracks extending parallel to one another, for instance in the form of a gray code.
Arranging the position information in a single code track, with code elements in line with one another in the measurement direction, is especially space-saving. The code elements are disposed in line with one another in a pseudo-random distribution, so that a certain number of successive code elements each form one bit pattern that unambiguously defines the absolute position. When the scanning instrument is shifted by a single code element, a new bit pattern is already formed, and over the entire measurement range to be detected in absolute form, a sequence of different bit patterns is available. This kind of sequential code is known as a chain code or a pseudo-random code.
In both multitrack and single-track absolute coding, it is a problem to achieve a space-saving layout on the one hand and high resolution on the other.
To enhance the resolution in sequential absolute coding, it is usual to provide at least one incremental graduation in addition. The period of this incremental graduation must be adapted to the length of a code element, or in other words to the maximum possible increment width or resolution of the code. As explained in German Patent Disclosure DE 41 23 722 A1, the graduation period or incremental graduation should be selected to exactly match the increment width of the code. To further enhance the resolution, a second incremental track is required, whose graduation period is a fraction of the graduation period of the first incremental track. To enhance the resolution of the absolute position measurement, that is, the subdivision of the increment width of the code, a plurality of incremental tracks disposed side by side are thus necessary.
This arrangement has the disadvantage that a space-saving layout is not possible, and that the scanning elements of the two incremental tracks are spaced apart from one another perpendicular to the measurement direction, and as a consequence the arrangement is vulnerable to rotations of the scanning unit (known as moiré fluctuations). Upon rotations between the scanning unit and the incremental graduations, the requisite synchronization of the scanning signals, derived from the two incremental tracks, is no longer assured.
For this reason, European Patent Disclosure EP 1 111 345 A2 has proposed disposing, besides the absolute code, only a single incremental graduation, which however has two different graduation periods. A detector arrangement is designed to generate a first incremental signal with a coarse signal period and a second incremental signal with a fine signal period, that is, incremental signals that are required for synchronization in the apparatus of DE 41 23 722 A1. A prerequisite for this synchronization is an interpolation of the incremental signals with the coarse signal period, which is why EP 1 111 345 A2 lists various provisions for filtering, in order to derive an interpolatable incremental signal with the coarse signal period from the incremental track. The provisions for generating two highly interpolatable incremental signals of EP 1 111 345 A2 require relatively great effort and expense.